If an aircraft is operating IFR (part 91) with a current instr. rated
private pilot acting as PIC, can a non instrument rated private pilot
manipulate the controls? I don't see anything in the FARs that could
explicitly prohibit this type of operation.

For example if instrument rated pilot A takes a trip with non instr.
rated pilot B, could B take the controls during the trip? If so how
should such time be logged? I believe that person A would be Acting as
PIC but could not log the time as he is not sole control manipulator.
Person B is not rated so he probably couldn't log the time either, since
person A is not a instructor.